Glass ceramic material, method for producing the same and spark plug containing such a glass ceramic material

ABSTRACT

A glass ceramic, for use as a resistor or a gas-tight glass ceramic solder for use in a spark plug, includes a fused seal of a starting glass fused from a starting mixture containing SiO 2 , Al 2 O 3 , TiO 2  and CaO, the fused seal including crystalline phases in at least some areas. A method for producing such a glass ceramic provides for the starting glass to be processed in a first method step to form a starting material, which is heated for a first period of time in a second method step from a starting temperature, which is below the softening temperature of the starting glass, to a fusion temperature, which is above the softening temperature of the starting glass, and is kept at that temperature for a second period of time and finally is cooled again. A spark plug may include a terminal stud and a center electrode, which are electrically connected across a resistor that is formed in at least some areas by the glass ceramic.

FIELD OF THE INVENTION

The present invention relates to a glass ceramic, which may be used, inparticular as a resistor or as a gas-tight ceramic solder in a sparkplug, a method of producing the glass ceramic and a spark plug havingsuch a glass ceramic.

BACKGROUND INFORMATION

German Published Patent Application No. 196 51 454 refers to a sparkplug including a terminal stud connected to a center electrode across aresistor in the form of a fused glass seal arranged between theelectrode and the terminal stud. This resistor seal may be made of aglass material or a glass ceramic material, which may be provided with ametal phase in the form of a network to increase the electricconductivity. This metal phase may be achieved by a surfacemetallization of glass powder deposited in a currentless operation andthen fused in the spark plug to form a resistor.

German Published Patent Application No. 196 23 989 refers to a sparkplug in which the terminal stud is connected to the center electrode bya burn-off resistor and a contact pin is arranged between the burn-offresistor and the center electrode.

SUMMARY OF THE INVENTION

An exemplary glass ceramic according to the present invention isbelieved to be suitable for use as a glass ceramic seal in a spark plug,in which the seal may have either a high or low resistance, depending onits composition. This exemplary glass ceramic may be stable attemperatures up to more than 1000° C., so that it may be used as aburn-off resistor in a spark plug.

This exemplary glass ceramic may be suitable for use in spark plugshaving a platinum center electrode, which may be heated up to 950° C. inthe area of the insulator base during operation. This exemplary glassceramic may tolerate an operating temperature of 900° C. for more than2000 hours.

Another exemplary glass ceramic according to the present invention maybe fused from a glass powder or a glass powder mixture at a processtemperature of less than approximately 950° C., including a temperaturebetween approximately 850° C. and 950° C., without requiring the use ofa protective gas. This exemplary glass ceramic may have high voltagestrength up to 20 kV/mm at room temperature or up to 10 kV/mm at 800°C., a thermal expansion coefficient of the glass ceramic being adaptedto that of aluminum oxide (Al₂O₃), which may be an insulator materialfor use in spark plugs. The thermal expansion coefficient may beapproximately 6 ppm/K from 100° C. to 200° C. and approximately 9 ppm/Kfrom 700° C. to 800° C. On the basis of these properties, an exemplaryglass ceramic of the present invention is believed to be suitable forproducing a glass ceramic seal as a burn-off resistor in a spark plughaving an adjacent insulator made of dense, pure crystalline aluminumoxide.

The high temperature stability and high voltage strength may be achievedbecause an exemplary glass ceramic according to the present inventionmay have refractory phases at least partially or at least in some areas,for example, the phases anorthite, wollastonite and titanite.

Through controlled processing of a starting glass to a startingmaterial, an exemplary glass ceramic may be produced according to thepresent invention in the form of an electrically conducting glassceramic solder. The exemplary glass solder may be used, for example, tocontact a glass ceramic resistor seal having a different composition toa metal, such as a contact pin, a stud or a center electrode in a sparkplug.

An exemplary glass ceramic according to the present invention may atleast better guarantee thermally stable and gas-tight contacting of thecenter electrode or the stud in a spark plug by varying its composition.

With regard to high voltage strength, thermal stability and adaptationof the thermal expansion coefficient to the surrounding insulator madeof, for example, aluminum oxide, an exemplary glass ceramic according tothe present invention may have a composition including a startingmixture containing approximately 43 wt % to 48 wt % SiO₂, approximately16.5 wt % to 18 wt % Al₂O₃, approximately 6 wt % to 10.5 wt % TiO₂,approximately 0.3 wt % to 1.2 wt % Na₂O, approximately 0.3 wt % to 1.2wt % K₂O and approximately 24.5 wt % to 28.5 wt % CaO.

Another exemplary glass ceramic according to the present invention, withregard to the development of refractory phases through a controlledtemperature treatment of the starting glass, may be obtained by a glassceramic having a composition including a starting mixture composed ofapproximately 45 wt % SiO₂, approximately 17 wt % Al₂O₃, approximately 9wt % TiO₂, approximately 0.5 wt % Na₂O, approximately 0.5 wt % K₂O andapproximately 28 wt % CaO.

To adjust electric properties of an exemplary glass ceramic according tothe present invention, two different alternatives may be used. First, ametal phase may be developed, which may be embedded in the glass ceramicand which may be a network. Second, a carbon phase may be developed,which may be embedded in the glass ceramic and which may be a network.The carbon phase may be composed of pyrolyzed carbon.

To produce another exemplary glass ceramic according to the presentinvention having desired crystalline phases, the fusion temperature ofthe starting material used may be between 850° C. and 950° C. The heatedstarting material may then be kept at this temperature for a firstperiod of time, which may be from 5 to 15 minutes, whereupon therefractory phases may be formed, and the starting material may beconverted into a glass ceramic. A glass ceramic is understood in thisconnection to be a material which, unlike a glass, has crystallinephases in at least some areas.

To produce yet another exemplary glass ceramic according to the presentinvention having a low resistance, for example, for use as a glassceramic solder, the glass powder used in an exemplary method accordingto the present invention may be provided with a metallization composedof a metal with high temperature stability or, alternatively, a compoundincluding a glass powder, a binder and a carbon black powder.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a sectional view of a spark plug having an exemplary glassceramic seal as a burn-off resistor.

FIG. 2 shows a sectional view of a spark plug having another exemplaryglass ceramic seal as a burn-off resistor.

DETAILED DESCRIPTION

To create a glass ceramic seal as a burn-off resistor or as a ceramicsolder for use in a spark plug 1, a starting glass is fused from astarting mixture, which may be composed of approximately 38 to 48 wt %SiO₂, approximately 15 to 19 wt % Al₂O₃, approximately 4.5 to 11 wt %TiO₂, approximately 0 to 1.5 wt % Na₂O, approximately 0 to 1.5 wt % K₂Oand approximately 23 to 30 wt % CaO. In addition, Li₂O may be added tothe starting mixture in an amount of up to approximately 1.5 wt %.

The starting mixture may be composed of approximately 43 to 48 wt %SiO₂, approximately 16.5 to 18 wt % Al₂O₃, approximately 6 to 10.5 wt %TiO₂, approximately 0.3 to 1.2 wt % Na₂O, approximately 0.3 to 1.2 wt %K₂O and approximately 24.5 to 28.5 wt % CaO. It is believed that bestresults may be obtained with a the starting mixture composed of 45 wt %SiO_(2,) 17 wt % Al₂O₃, 9 wt % TiO₂, 0.5 wt % Na₂O, 0.5 wt % K₂O and 28wt % CaO.

To produce a burn-off resistor in the form of a glass ceramic resistorseal, which may be connected by a glass ceramic solder to adjacentmetallic parts of the spark plug, the starting mixture described abovemay first be fused to form a starting glass, which may then be splitinto two batches. The first batch of this starting glass may be groundto form a glass powder having a mean particle size of approximately 150μm to 250 μm.

An exemplary glass powder prepared in this way may be used as an actualfused high-resistance resistor seal in the form of a burn-off resistorfor use in a spark plug.

The second batch of the starting glass may be used to produce a glassceramic solder for joining the burn-off resistor to adjacent metal partsin the spark plug.

To produce this glass ceramic solder in the form of a low-resistanceglass ceramic, two alternatives may be used.

A first exemplary embodiment provides for a starting glass to be groundto form a glass powder having a mean particle size of less thanapproximately 250 μm. The starting glass may then be provided with asurface metallization by a currentless metallization method, such asthat referred to in German Published Patent Application No. 196 51 454.The surface metallization may include a metal having a high-temperaturestability, such as platinum, palladium, nickel, tungsten or an alloy ofthese metals. Palladium may be beneficial. The thickness of the surfacemetallization may be from approximately 0.5 nm to 10 nm, and inparticular from 2 nm to 5 nm.

A second exemplary embodiment provides for the starting glass to beprocessed to form a starting material in a first method step. A firstportion of the starting glass may be ground to form a first glass powderhaving a mean particle size of approximately 150 μm to 250 μm, and asecond portion of the starting glass is ground to form a-second glasspowder having a mean particle size of less than approximately 100 μm, inparticular, from 5 μm to 70 μm. These two glass powders, havingdifferent particle sizes, may then be mixed with a carbon black powderand an organic binder. The carbon black powder may have a mean particlesize of approximately 200 nm to 2 μm, in particular, from 400 nm to 600nm. The organic binder may be a mixture of carboxymethylcellulose anddextrin, using water as a solvent. Zirconium dioxide powder and mullitepowder may be added to the starting material, each possibly having amean particle size of less than approximately 100 μm.

The starting glass may be processed in a first method step to form astarting material containing an amount of approximately 40 wt % to 58 wt% of the first glass powder, an amount of approximately 3 wt % to 13 wt% of the second glass powder, an amount of approximately 0.9 wt % to 2.5wt % of the carbon black powder, an amount of approximately 10 wt % to37 wt % of the zirconium dioxide powder, an amount of approximately 8 wt% to 13 wt % of the mullite powder and an amount of approximately 0.6 wt% to 4 wt % of the organic binder. The values given in wt % above arebased on a solvent-free starting material, to which a solvent issubsequently added, for example, water. The amount of the solvent may beapproximately 12 to 40 vol %,for example, approximately 22 to 37 vol %.

After the starting glass has been prepared in the form of a glass powderand either a surface-metallized glass powder or a processed startingmaterial of a glass powder mixture has been prepared, these materialsmay then be used in the production of spark plug 1.

FIG. 1 shows a sectioned view of a spark plug 1, conventional sparkplugs being referred to in German Published Patent Application No. 19651 454, and has an insulator 2 made, for example, of pure, crystallineand gas-tight aluminum oxide, a metallic terminal stud 3, a housing 4, aresistor 5 in the form of a glass ceramic resistor seal, a centerelectrode 6 which may, for example, be made of platinum or coated withplatinum, and a ground electrode 7. Terminal stud 3 includes a lower end8.

Between lower end 8 of terminal stud 3 and resistor 5, a gas-tight glassceramic solder 9 is provided, which connects resistor 5 and terminalstud 3, so they are gas-tight. A gas-tight glass ceramic solder 9 isprovided between center electrode 6 and resistor 5, which connectsresistor 5 to center electrode 6, so they are gas-tight.

Spark plug 1 has an upper end 10, a thread 11 and a terminal nut 12 forconnecting an ignition line. In addition, a leakage current barrier 13and a polygonal arrangement 16 are provided, with the help of whichspark plug 1 may be screwed into an engine block. A thread 17 isprovided for this purpose. Center electrode 6 and ground electrode 7 areseparated by a spacer 18. Insulator 2 protrudes to a great extent,shielding center electrode 6 from ground electrode 7 in some areas. Forthis purpose, insulator 2 has an insulator base tip 19, so that a sparkgap can be established only between the tip of center electrode 6 andground electrode 7. Spark plug 1 has an end face 20 which limits theextent to which spark plug 1 can be screwed into an engine block.

To produce the spark plug of FIG. 1, insulator 2 may be first providedwith center electrode 6 in an available manner. Then, either the glasspowder that has been provided with a surface metallization or theprepared starting material composed of the glass powder mixture, carbonblack, organic binder and optionally other ingredients may be packedinto the cavity in insulator 2 as glass ceramic solder 9. To producehigh-resistance resistor 5, the starting glass, which has been ground toform a glass powder, may be packed into insulator 3. Either thesurface-metallized glass powder or the starting material that has beenprepared in the manner described above, may be added on top of theground starting glass. Terminal stud 3 may be placed on top of thissequence of layers.

The second method step for producing an exemplary glass ceramic providesfor the different materials introduced into the insulator to be heatedfor a first period of time from a starting temperature, which may bebelow the softening temperature T_(g) of the starting glass, to a fusiontemperature, which may be above the softening temperature T_(g) of thestarting glass, to form resistor 5 or gas-tight glass ceramic solder 9together with the spark plug and inserted stud 3. Spark plug 1 may thenbe kept at this temperature for a second period of time and then cooledagain.

The starting temperature in the heat treatment described above may bebetween 10° C. and 40° C., and which may be room temperature. The fusiontemperature may be approximately 850° C. to 950° C. The first period oftime during which the spark plug is heated together with the addedstarting materials may be between 5 and 15 minutes, for example, between8 and 10 minutes. The second period of time during which the spark plugis kept at the fusion temperature may be between 5 and 25 minutes, forexample, between 9 and 15 minutes. In the course of this heat treatment,i.e., during the fusion operation, crystallization may occur at leastpartially and/or at least in some areas in the starting glass. This mayresult in the formation of, among other things, refractory phasesanorthite (CaO·Al₂O₃·2SiO₂), wollastonite (CaO·2SiO₂) and titanite(CaO·TiO₂·SiO₂).

After this heat treatment, a seal may be formed between center electrode6 and stud 3 in the form of a glass ceramic which may have a resistanceof more than 1 kΩ in the area of resistor 5, the glass ceramic forming ahigh-resistance seal as a burn-off resistor. Two areas may be formedwith a gas-tight glass ceramic solder 9 connecting resistor 5 to centerelectrode 6 and stud 3. Depending on the composition selected, gas-tightglass ceramic solder 9 may be a glass ceramic including a metal phaseembedded in it, for example, a metal phase in the form of a network, inat least some areas, so that this metal phase essentially carries theelectric conductivity of the glass ceramic solder. The glass ceramicsolder 9 may be in the form of a glass ceramic having a carbon phaseembedded in it in at least some areas, for example, a carbon phase inthe form of a network. This carbon phase may be formed by pyrolysis fromthe organic binder and/or carbon black powder added to the preparedstarting material.

Glass transition temperature T_(g) of the starting glass may be 753° C.,but it may also be between 670° C. and 780° C., depending on thecomposition of the starting glass. The dilatometric softeningtemperature E_(g) of the starting glass thus produced may be 786° C.,but it may also vary between approximately 720° C. and 820° C.,depending on the composition of the starting glass. The thermalexpansion coefficient α of the starting glass may be approximately8.3×10⁻⁶/K at a temperature of 100° C. to 500° C., but it may also varybetween 6.7×10⁶/K and 8.8×10⁶/K, within the scope of the limits asreferred to above.

FIG. 2 shows an alternative embodiment of spark plug 1. The spark plugof FIG. 2 differs (see German Published Patent Application No. 196 23989) from the spark plug of FIG. 1, including based on the shapes ofground electrode 7 and center electrode 6. In addition, resistor 5 isset back in comparison with that of FIG. 1, with a terminal stud 21being provided between resistor 5 and center electrode 6. Production ofthe spark plug of FIG. 2 is analogous to that in FIG. 1, with regard tothe design of resistor 5 and gas-tight glass ceramic solder 9. Accordingto FIG. 2, resistor 5 is not connected on its lower side to centerelectrode 6, however, and instead is connected in a gas-tight manner toterminal stud 21 by glass ceramic solder 9.

What is claimed is:
 1. A glass ceramic, comprising: a starting glassfused from a starting mixture containing SiO₂, Al₂O₃, TiO₂, and CaO,wherein the glass ceramic is a fused seal of the starting glass and hascrystalline phases in at least some areas; wherein the starting mixturecontains approximately 38 wt % to 48 wt % of the SiO₂, approximately 15wt % to 19 wt % of the Al₂O₃, approximately 4.5 wt % to 10 wt % of theTiO₂, approximately 0 wt % to 1.5 wt % of Na₂O, approximately 0 wt % to1.5 wt % of K₂O and approximately 23 wt % to 30 wt % of the CaO.
 2. Theglass ceramic of claim 1, wherein the starting mixture includes lithiumoxide in an amount of up to approximately 1.5 wt %.
 3. A glass ceramic,comprising: a starting glass fused from a starting mixture containingSiO₂, Al₂O₃, TiO₂, and CaO, wherein the glass ceramic is a fused seal ofthe starting glass and has crystalline phases in at least some areas;wherein the starting mixture contains approximately 43 wt % to 48 wt %of the SiO₂, approximately 16.5 wt % to 18 wt % of the Al₂O₃,approximately 6 wt % to 10.5 wt % of the TiO₂, approximately 0.3 wt % to1.2 wt % of Na₂O, approximately 0.3 wt % to 1.2 wt % of K₂O andapproximately 24.5 wt % to 28.5 wt % of the CaO.
 4. A glass ceramic,comprising: a starting glass fused from a starting mixture containingSiO₂, Al₂O₃, TiO₂, and CaO, wherein the glass ceramic is a fused seal ofthe starting glass and has crystalline phases in at least some areas;wherein the starting mixture contains approximately 45 wt % of the SiO₂,approximately 17 wt % of the Al₂O₃, approximately 9 wt % of the TiO₂,approximately 0.5 wt % of Na₂O, approximately 0.5 wt % of K₂O and 28 wt% of the CaO.
 5. A glass ceramic, comprising: a starting glass fusedfrom a starting mixture containing SiO₂, Al₂O₃, TiO₂, and CaO, whereinthe glass ceramic is a fused seal of the starting glass and hascrystalline phases in at least some areas; wherein the glass ceramicincludes refractory phases anorthite, wollastonite and titanite.
 6. Theglass ceramic of claim 5, wherein the glass ceramic includes in at leastsome areas a metal phase embedded in the glass ceramic in a networkform.
 7. A glass ceramic, comprising: a starting glass fused from astarting mixture containing SiO₂, Al₂O₃, TiO₂, and CaO, wherein theglass ceramic is a fused seal of the starting glass and has crystallinephases in at least some areas; wherein the glass ceramic includes in atleast some areas a carbon phase embedded in the glass ceramic in anetwork form.
 8. The glass ceramic of claim 1, wherein starting glassincludes a glass powder having a mean particle size of less than 250 μm.9. A method of producing a glass ceramic, comprising: processing astarting glass fused from a starting mixture containing SiO₂, Al₂O₃,TiO₂, and CaO to form a starting material; heating the starting materialfor a first time period from a starting temperature that is below asoftening temperature of the starting glass to a fusion temperature thatis above the softening temperature of the starting glass; maintainingthe starting material at the fusion temperature for a second timeperiod; and cooling the starting material; wherein the glass ceramic isa fused seal of the starting glass and has crystalline phases in atleast some areas, wherein the step of processing the starting glassincludes milling the starting glass to form a glass powder having a meanparticle size of less than 250 μm, and wherein the glass powder isprovided at least partially with a surface metallization in the step ofprocessing.
 10. The method of claim 9, wherein the starting temperatureis between approximately 10° C. and 40° C. and the fusion temperature isbetween approximately 850° C. and 950° C.
 11. The method of claim 9,wherein the first time period is between approximately 5 and 15 minutes,and the second time period is between approximately 5 and 25 minutes.12. A method of producing a glass ceramic, comprising: processing astarting glass fused from a starting mixture containing SiO₂, Al₂O₃,TiO₂, and CaO to form a starting material; heating the starting materialfor a first time period from a starting temperature that is below asoftening temperature of the starting glass to a fusion temperature thatis above the softening temperature of the starting glass; maintainingthe starting material at the fusion temperature for a second timeperiod; and cooling the starting material; wherein the glass ceramic isa fused seal of the starting glass and has crystalline phases in atleast some area, wherein the step of processing the starting glassincludes milling the starting glass to form a glass powder having a meanparticle size of less than 250 μm, and wherein the glass powder isprovided with a metallization selected from a group consisting of: ametal that is stable at high temperatures, platinum, palladium, nickel,tungsten, and an alloy having a thickness of 0.5 nm to 10 nm.
 13. Amethod of producing a glass ceramic, comprising: processing a startingglass fused from a starting mixture containing SiO₂, Al₂O₃, TiO₂, andCaO to form a starting material; heating the starting material for afirst time period from a starting temperature that is below a softeningtemperature of the starting glass to a fusion temperature that is abovethe softening temperature of the starting glass; maintaining thestarting material at the fusion temperature for a second time period;and cooling the starting material; wherein the glass ceramic is a fusedseal of the starting glass and has crystalline phases in at least somearea; and wherein the step of processing the starting glass to form thestarting material includes milling a first portion of the starting glassto form a first glass powder having a mean particle size ofapproximately 150 μm to 250 μm, milling a second portion of the startingglass to form a second glass powder having a mean particle size of lessthan approximately 100 μm, and mixing the first glass powder and thesecond glass powder together with a carbon black powder and an organicbinder.
 14. The method of claim 13, wherein the carbon black powder hasa mean particle size of approximately 200 nm to 2 μm.
 15. The method ofclaim 13, wherein the organic binder includes a mixture ofcarboxymethylcellulose and dextrin, with water as a solvent.
 16. Themethod of claim 13, wherein the step of processing the starting glass toform the starting material includes adding zirconium dioxide having amean particle size of less than 100 μm.
 17. The method of claim 13,wherein the step of processing the starting glass to form the startingmaterial includes adding mullite.
 18. The method of claim 13, whereinthe starting material contains an amount of approximately 40 wt % to 58wt % of the first glass powder, an amount of approximately 3 wt % to 13wt % of the second glass powder, an amount of approximately 0.9 wt % to2.5 wt % of the carbon black powder, an amount of approximately 10 wt %to 37 wt % of zirconium dioxide, an amount of approximately 8 wt % to 13wt % of mullite and an amount of approximately 0.6 wt % to 4 wt % of theorganic binder, the amounts in wt % being based on a solvent-freestarting material.
 19. The method of claim 13, wherein a solvent isadded to the starting material in an amount of approximately 12 to 40vol %.
 20. A spark plug, comprising: a resistor made in at least someareas of a glass ceramic, wherein the glass ceramic includes a startingglass fused from a starting mixture containing SiO₂, Al₂O₃, TiO₂, andCaO, and the glass ceramic forms a fused seal of the starting glass andhas crystalline phases in at least some areas of the glass ceramic; anda terminal stud and a center electrode electrically connected across theresistor.
 21. The spark plug of claim 20, further comprising a contactpin arranged between the resistor and the center electrode.
 22. Thespark plug of claim 20, wherein the resistor is connected to at leastone of the terminal stud and the center electrode by a glass ceramicsolder made of a glass ceramic including a starting glass fused from astarting mixture containing SiO₂, Al₂O₃, TiO₂, and CaO, the glassceramic forming a fused seal of the starting glass and havingcrystalline phases in at least some areas.
 23. The spark plug of claim21, wherein the resistor is connected to at least one of the terminalstud and the contact pin by a glass ceramic solder made of a glassceramic including a starting glass fused from a starting mixturecontaining SiO₂, Al₂O₃, TiO₂, and CaO, the glass ceramic forming a fusedseal of the starting glass and having crystalline phases in at leastsome areas.
 24. The glass ceramic of claim 1, wherein the startingmixture includes lithium oxide in an amount of up to approximately 1.5wt %.
 25. The method of claim 9, wherein the first time period isbetween approximately 8 and 10 minutes, and the second time period isbetween approximately 9 and 15 minutes.
 26. The method of claim 13,wherein the second glass powder has a mean particle size ofapproximately 5 μm to 70 μm.
 27. The method of claim 13, wherein themean particle size of the carbon black powder is approximately 400 nm to600 nm.
 28. The method of claim 13, wherein a solvent is added to thestarting material in an amount of approximately 22 vol % to 37 vol %.